There are as many theories as to what has gone wrong for Manchester United this season as there have been home defeats and another one emerged in the press-conference Roy Hodgson gave after England's 1-0 friendly victory over Denmark.
Having spoken about the pace and energy of his young players and the need to get them running in behind defenders, the England manager complained of the "ponderousness" of the first-half display.
Given how often Wayne Rooney had lumbered into decent positions and then blootered the ball over the bar, it was possible in context to wonder if he was expressing an irritation with the Manchester United forward.
Then a couple of minutes later, Hodgson was far more explicit, speaking of the lack of pace in the game "in the half that Rooney played."
Hodgson has a habit of talking himself into trouble, of vowing not to broach a topic before finding his tongue has got away from him and he has discussed it after all: the explanation here seemed to be that he was troubled by the lack of urgency while Rooney was on the pitch and that his puzzlement leaked out, presumably unintentionally.
It may not even be that he blamed Rooney for it, for when Rooney had been on the pitch, England had played an asymmetric 4-3-3 in which Daniel Sturridge and Rooney had taken it in turns to operate as the central striker, a sharing of roles in which neither seemed comfortable.
The comment could very easily have been part of Hodgson's thinking on whether the shape needed shifting so Rooney played deeper.
But it did raise the thought that perhaps there was a ponderousness about Rooney more generally these days and that in that might lie some of the explanation for Manchester United's struggles. These things, of course, are never simple: does Rooney's form destabilise United or does United's form destabilise Rooney, or—more likely—is it both?
Rooney's performance against West Bromwich Albion on Saturday was good enough to suggest the fundamental problem does not lie with him. He scored one and made one and at times seemed to be dragging United forward single-handed.
If there is an issue, it's his relationship with Robin van Persie, which is pretty much non-existent. They are two very good players who play up front at the same time, but they are not a partnership, very rarely linking up.
When Van Persie made his complaint after the defeat away to Olympiakos about other players running in his zones, it was pretty clear it was Rooney he was thinking about primarily.
Gerrard is still far from the finished product as a holding midfielder—although he has improved a lot in the deep role this season—and may struggle to contain Rooney, but then Rooney also has a defensive job to do: it's been notable that when Liverpool have been at their best this season, as in that first 20 minutes at home to Arsenal, Gerrard has had freedom to dictate the play from deep.
Rooney is one of the best defensive forwards in the game. It's a part of the game he seems to enjoy less and less, but the stats show he has been doing it this season.
To be at his most effective Rooney needs not to stop Gerrard as well as offering a creative threat.
Whether he performs his defensive functions is largely down to him, but to be creative he needs service and players making runs off him—the absence of which has been one of United's problems this season as the emphasis on crosses has increased.
Rooney cannot beat Liverpool single-handed, but if they are to triumph, he will probably have played a key role.